1. Field of the Invention
The present invention relates to an optical switch utilizing thermooptic effects. This invention is based on a Japanese Patent Application No. 2000-28145, the contents of which are incorporated herein by reference.
2. Background Art
Cross connection technology is a key technology in the next generation optical communication network. It is expected that a variety of optical switches will be utilized in the coming optical communication network.
As an example, optical switches based on thermooptic effects are known, and many operational schemes have been proposed.
FIG. 7A shows a plan view of such an optical switch, and FIG. 7B is a cross sectional view of the switch through a plane Axe2x80x94A in FIG. 7A.
In the diagram, reference numeral 2 relates to a substrate base upon which a cladding layer 3 is formed, and a Y-shaped core 4 is disposed in an interior of the cladding layer 3. This type optical waveguide is what is called xe2x80x9ca buried channel optical waveguidexe2x80x9d.
Silicon substrate etc., for example, may be used for the substrate base 2. The cladding layer 3 and the Y-shaped core 4 are made of a transparent material.
The Y-shaped core 4 is made of a material having a higher refractive index than the cladding layer 3 so as to act as a optical waveguide. Also, as will be described later, the Y-shaped core 4 is preferably made of a polymeric material (plastic) so that the refractive index can be altered by application of heat.
A polymeric material similar to the Y-shaped core 4 is used preferably as the material for the cladding layer 3.
The Y-shaped core 4 is formed in such a way that a core having a cross sectional shape of a square-rod is split at a portion along length of the core into two branched cores. The Y-shaped core 4 is comprised by an input-side linear section 4a extending from the input side; a branching section 4b formed on the output side of the input-side linear section 4a, in which the width of the input-side linear section 4a increases gradually; a separation section 4c formed in a curved or linear shape so that the two branched cores 5a, 5b separate from each other as they extend from the branching section 4b; and an output-side linear section 4d in which the branched cores 5a, 5b extend parallel to each other.
In the branching section 4b, the input-side linear section 4a extends from the input end of the branching section 4b, and the branched cores 5a, 5b extend from the bottom perimeter opposite to the input end of the branching section 4b. 
An input port 6a on the input side of the Y-shaped core 4 and two ports on the output side of the Y-shaped core 4 are placed coplanarly on a plane parallel to the bottom and top surfaces of the substrate base 2.
On top of the cladding layer 3, line heaters 7, 8 comprised by an electrically conductive thin film such as titanium, gold or aluminum etc. are placed so as to extend longitudinally along and on the outside of the Y-shaped core 4, part way from the input-side linear section 4a through the branching section 4b to the separation section 4c. Heater 7 is disposed on the branched core 5a side, and heater 8 is disposed on the branched core 5b side.
On both end sections of the heaters 7, 8, rectangular-shaped electrode pads 7a and electrode pads 8a are disposed, respectively, on the outer side of the Y-shaped core 4, and are connected to respective external electrodes. Electrode pads 7a and electrode pads 8a are formed as thin films and is made of a material similar to the heaters 7, 8.
Also, in the input-side linear section 4a, heaters 7, 8 are disposed away from the optical path with a suitable separation distance while in the branching section 4b and the separation section 4c, the heaters 7, 8 are disposed in close proximity to the optical path.
Therefore, if the electrical power is supplied only to heater 7, the branched core 5a side of the branching section 4b and the branched core 5a in the separation section 4c are heated. Rise in temperature causes the effective refractive index to decrease due to thermooptic effects. The result is that light is output from the branched core 5b by propagating through the branched core 5b side of the branching section 4b which is not being heated. In other words, propagation of light through the branched core 5a is selectively blocked.
On the other hand, if the electrical power is supplied only on heater 8, the branched core 5b side of the branching section 4b and the branched core 5b in the separation section 4c are heated, so that light is output from the branched core 5a by propagating through the branched core 5a side of the branching section 4b which is not being heated. In other words, propagation of light through the branched core 5b is selectively blocked.
The result is that when heater 7 is activated, a light inputted into input port 6a is output from port 6c through the branched core 5b, and when heater 8 is activated, light inputted into port 6a is output from output port 6b through the branched core 5a. 
Then, by changing the action to heat the branching section 4b and the branched cores 5a by operating the heater 7 and the action to heat the branching section 4b and the branched cores 5b by operating the heaters 8, the optical path can be altered to obtain an optical switch functioning such that a light input into port 6a can be output at will from either output port 6b or 6c. 
In such an optical switch, in order to guide the light from the branching section 4b to either the branched core 5a or the branched core 5b, it is necessary to adjust the temperature distribution (i.e., refractive index distribution) suitably in the branching section 4b by heating either heater 7 or heater 8.
If the heating temperature is too low, it is not possible to create a sufficient change in the refractive index in the heated section of the branching section 4b. The result is that light is transmitted through the heated section so that the light reaches the heated branched core, thereby generating an insertion loss.
Conversely, if the temperature is too high, even the refractive index in the side of the branched core intended for light output of the branching core 4b becomes affected so that the light cannot reach the branched core intended for light output, leading to an increase in the insertion loss.
On the other hand, the branched core intended for light-blocking must be heated sufficiently so as to not to permit light to be output from its port.
However, in this type of optical switch, because the branching section 4b and the separation section 4c are heated as a unit, it is experienced sometimes that if the temperature distribution in the branching section 4b is adjusted suitably, the separation section 4c cannot be heated sufficiently, and conversely, if the heating condition in the separation section 4c is adjusted suitably, appropriate temperature distribution in the branching section 4b could not be obtained.
Therefore, adjustment of heating conditions has been troublesome, and it has been difficult to reduce insertion losses.
The purpose of the present invention is to reduce insertion loss in an optical switch based on thermooptic effects.
Specifically, an object of the invention is to provide an optical switch that enables to adjust the temperature distribution in the branching section of a core and to apply optimal heat to branched cores.
To achieve the object, the present optical switch is comprised by a cladding layer and a core disposed in an interior of the cladding layer for light propagating in such a way that a width of the core is enlarged at a branching section formed at a portion along length of the core to provide plural branched cores to enable to alter a propagation path of inputted light by selective heating of portions of the branching section and the plural branched cores, wherein a branching section heater for heating the branching section and branched core heaters for heating the plural branched cores are controlled separately.
In the optical switch according to the present invention, because the heater for heating the branching section is controlled separately from the heaters for heating the plural branched cores, the branching section and the branched cores can be controlled individually at respective temperatures for their optimal performances. The result is that the insertion loss is reduced and it is possible to block outputting light from a port not intended for light output.